Customization of Python2, Python3, IPython executable

While `py-shell-name' considered holding the default

py-python-command - defaults to python2
py-python2-command
py-python3-command
py-ipython-command

will receive defaults depending from system

Respective

py-python-command-args
py-python2-command-args
py-python3-command-args
py-ipython-command-args

That way

M-x python RET
M-x python2 RET
M-x python3 RET
M-x ipython RET

should see reasonable defaults on all systems